The present invention relates to an apparatus and method for the wrapping of an object by enclosing it in a film of stretched material, normally plastics material. It is particularly concerned with wrapping bales of grass.
Stretch wrapping is well-known, and various wrapping apparatuses have been devised to carry it out. For example in GB No. 2063809 a stretch wrapping apparatus is disclosed in which web material is passed over two rollers rotating at different speeds, so that the web is stretched between the rollers, and then wrapped around an object. (U.S. Pat. Nos. 4,302,920, 4,336,679, 4,387,548 and 4,429,514 correspond, and their disclosures are incorporated herein by reference.)
FIG. 1a of the accompanying drawings shows the stretch wrapping apparatus 10 of GB No. 2063809. This has a horizontal frame 11 on which an object 12 to be wrapped is mounted via a rotatable turntable 13, and a vertical frame 14 on which is mounted the device for stretching the film. A roll 15 of film to be stretched is mounted on a mandrel rotatable about an axis 16, and the film web is drawn from that mandrel round a first roller 17 and a second roller 18 spaced apart by a distance 19. From the roller 18, the web 20 extends to the object 12 (optionally via an idle roller 21). The turntable 13 rotates the object 12 so that the web 20 is drawn from the roll 15 to the object 12, via the rollers 17, 18. These are arranged to rotate at different speeds, the roller 18 travelling faster than the roller 17, so that the web 20 is stretched as it passes over the distance 19.
In normal use, the optimum relationship between the speeds of rotation of the rollers 17 and 18, which determines the elongation of the stretched web, is determined by the type of material being stretched. Typical materials are polyethylene, and in particular Linear Low Density Polyethylene (LLDPE) which has improved stretch characteristics relative to normal polyethylene, or PVC.
In normal industrial settings, the apparatus of GB No. 2063809 discussed above with reference to FIG. 1 has proved satisfactory. However, the applicants have considered the wrapping of cut grass in this way and have discovered problems.
It is now becoming common for grass for silage to be cut and then rolled into cylindrical bales which are then stored. To protect the bales, it is known to wrap them in plastic, and it has been perceived that stretch wrapping techniques would be applicable. However, it has been found that slippage of the film occurs at the rollers, so that parts of the web are not stretched fully. This has the disadvantage of requiring more material than necessary to wrap a given bale, and also causes the problem that when the plastics material shrinks, as it does to a limited extent due to recovery of shape, the shrinkage will be uneven around the bale. The reason for the slippage is not fully understood. It is thought, however, that grass debris collects on the surfaces of the rollers, effectively lubricating the surfaces.
In GB No. 2063809 it had been proposed that the rollers were rubber faced, as it was thought that maximum film contact was preferred.